Other mistake: Chambers asks what time it is on Earth, a strangely vague question to which the Kanamits grant an equally odd answer: on Earth, it is noon. It can't be noon everywhere on Earth at once. While it can be fancifully explained away ("They somehow knew to answer for the time zone he was taken from"), it still jars the viewer, because neither Chambers nor the aliens, both highly intelligent, should make this sort of imprecise mistake. Both director Richard Bare and Damon Knight, author of the original short story, confirmed this as a scriptwriting error that should have been caught before shooting began. (00:02:15)
Factual error: Author Damon Knight was happy with Rod Serling's TV adaptation of his short story - except for the change that allowed the humans to translate the Kanamit language as though it were a code. Said Knight, "Without some sort of interplanetary Rosetta Stone, deciphering an unknown language would be impossible." (00:22:10)
Revealing mistake: Stock footage used here is far too obviously borrowed from several old movies. While the episode is set in the 60s, people's clothing and all the cars are noticeably many years out of date. (00:03:05)
Factual error: As well as the obvious problem with translating an unknown language as if it was a code, there is no chance that the word "serve" would have the same double meaning in English and Kanamit. It would be like the word "dope" and its French equivalent meaning "idiot" and "illegal drugs" in both languages.
Other mistake: The Kanamit have to lower their heads to pass through the doorways on their own ship.
Suggested correction: Just as astronauts on the International Space Station and the Shuttle before that had to.
